1. Field of the Invention
The present invention relates to a non-aqueous type electric double layer capacitor which has a high power output and a high energy density.
2. Discussion of Background
Heretofore, as a separator to be disposed between positive and negative electrodes of an electric double layer capacitor, a polyethylene non-woven fabric, a polypropylene non-woven fabric, a polyester non-woven fabric, kraft paper, a rayon/sisal mixed sheet, a manila hemp sheet or a glass fiber sheet is, for example, known (e.g. JP-A-9-45586, JP-A-1-304719). The role of a separator is to electrically insulate the positive electrode from the negative electrode on one hand and to facilitate transfer of ions in the electrolyte, which takes place accompanying charge and discharge, on the other hand.
In recent years, an attention has been drawn to an electric double layer capacitor for high power applications. However, with a separator made of organic fibers e.g. a polyethylene, the ion conductivity is low, and the internal resistance of the electric double layer capacitor is high, since the liquid absorbing property and the liquid-holding property for the electrolyte are low. Accordingly, if instantaneous large current discharge was carried out as one of main characteristics of an electric double layer capacitor, the voltage drop was substantial, such being not practical.
Further, a conventional separator made of paper is excellent in heat resistance and tensile strength and is sometimes effective for an electric double layer capacitor to be used for a power source wherein no large current discharge is carried out, like a hybrid power source used with a solar cell. However, when a conventional separator made of paper is used for an electric double layer capacitor for high power applications, wherein the electrolyte is non-aqueous, the ion permeability tends to be inadequate.
With an electric double layer capacitor, it is desired to lower the resistance and to increase the capacitance per unit volume, and it is accordingly required to make the separator as thin as possible. However, if a separator made of paper is made thin, the insulating property between the positive and negative electrodes tends to be inadequate, thus leading to micro-short circuiting, serious self-discharge.